Method of forming oxide coating on tantalum electrodes



Patented Aug. 21, 1951 UNITED STATES PATENT OFFICE METHOD OF FORMING OXIDE COATING ON TANTALUM ELECTRODES 4 Earl K. Smith, West Orange, N. J., assignor to Electrons, Incorporated, Newark, N. J a corporation of Delaware No Drawing. Original application March 3, 1948,

Serial No. 12,726. Divided and this application September 10, 1949, Serial No. 115,104

3 Claims.

This invention relates to electron discharge tubes, and more particularly to a process for forming a surface coating for the electrodes of such tubes to facilitate heat radiation and reluster causing its surface to have a low heat radiation coefficient, so that tantalum. tends to assume a high temperature in the operation of the tube.

With these and other considerations in mind,

duce electron emission from the surface of such 5 the primary object of this invention is to proelectrode at a relatively high operating tBmvide a surface coating or treatment for tantalum Derature. electrodes to facilitate radiation of heat and re- One problem in the construction of electron duce electron emission, as compared with plain discharge tubes, more particularly for gas filled metallic tantalum. More specifically, this in-- tubes of the higher continuous current rating, vention relates to a process for treating tantais that the anode, and also the control electrode lum electrodes to form a uniform and consistent when used, tends to become overheated and emit oxidized surface, which has the desired qualities electrons to a degree to interfere with the proper of low emissivity and superior heat radiating p rformance of the tube. Amon o h r things, ability, without rendering the tantalum brittle excessive electron emission from the anode of or incapable of forming a strong weld. a gas filled tube tends to reduce the negative This application is a division of my prior apor inverse voltage which the tube will stand plication Ser. No. 12,726, filed March 3, 1948, now without an arc-back or ischar e through the U. :5. Patent No. 2,538,053, and relates to the tube in the wrong direction. process of forming a coating of an oxide of tan- Such objectionable electron emission from talum on the surface of a tantalum electrode, electrodes depends of course upon the temperaas originally disclosed and claimed in said prior ture the electrode assumes in operation of the application. 7 tube, and also the nature of its surface as an The surface treatment for a tantalum elecelectron emitter at such temperature level. The trode in accordance with this invention involves heat accumulated by electrodes during operation the application of heat to the electrode in such of the type of tube contemplated is dissipated a Way as to form on its exposed surface a film largely by radiation, assisted by conduction of a g f d Which pp y is heat to the outside of the tube envelope through one of the lower oxides of tantalum presumably the supporting elements for the electrode; and the tetroxide TazOi. It is found that the oxide from the standpoint of temperature rise of the fi m r i g resulting f m h oxid o electrode, it is desirable that it should have a process f this invention has h d i d hi h surface with good heat radiating qualities. Also, heat radiation coefiioient and low electron in addition to facilitating heat radiation from emissivity with the same operating temperature, the electrode to reduce its operating temperaas compared with a plain tantalum surface. ture, it is desirable that the electrode surface In forming this oxide coating for the plain should be such as to have minimum electron tantalum surface of the anode, this surface 'is emission at the temperature it assumes in operfirst thoroughly cleaned and degreased by any ation. one of the well known procedures, and then is In certain types of tubes, tantalum is preferasubjected to repeated cycles of alternately heatbly employed for electrode material, more par- 0 ing in a gas flame and cooling with a brief exticularly for the anode of gas filled tubes, priposure to the atmosphere until the desired coatmarily because tantalum has a marked affinity in h n f The p f r m thod for various gases other than the insert or rare of treatment involves heating the tantalum to gases such as xenon, used as the final gas filling a moderate temperature generally approxithe tube. For example, tantalum readily abmately a dull d heat in the 1101? p Of a sorbs small traces of gases such as carbon as fl m P e r b y ju t d to b S y monoxide, hydrogen and oxygen that may be oxidizing, moving the heated part of the tantagiven off during operation of the tube and might lum out of the hot part of the gas name into otherwise have a deleterious or poisonous eifect P H to an OXid ZiH atmosp e e. unt l t e upon electron emission from the cathode. temperature drops and the visible color disap- Hence tantalum is a desirable material for the pears, and then repeating this cycle several anode of hot cathode gas filled tubes. Tantalum, times until a coating of the desired thickness however, is a grayish white metal, and'in its and d s y i f usual commercial form, has a definite metallic It is found that this method of forming the surface coating gives a more consistent and uniform surface film and adequate bonding to the metal surface, without the tantalum becoming too brittle, than a single heating in a flame and exposure to the atmosphere, or by continuous heating in air by induction heating or the like. The reasons for this are not fully understood. In this connection, one characteristic of tantalum when heated in air to a dull red heat is that it absorbs nitrogen greedily and tends to become brittle, presumably on account of formation of an excessive amount of a nitride or other nitrogen tantalum compound; yet temperatures of that order are needed to assure the formation of an oxide of tantalum. It may be that the brief exposures to the air in connection with moving the tantalum into and out of the hot part of a gas flame, in accordance with the preferred method of treatment, tends to limit the absorption of nitrogen and the formation of tantalumnitrogen compounds tending to make the tantalum too brittle. There are indications that it may be desirable to form a nitride of tantalum to some extent for optimum heat radiation and minimum electron emission qualities of the surface coating, so that cyclic heating and brief exposures to the air in accordance with this invention is preferable to a process of oxidation in a nitrogen free environment. It may be added that the gas flame may contribute to the formation of the desired film in some manner not clearly disclosed or understood by my investigations to date, and also that it is likely that the repeated heating and exposure to the air tends to build up the desired thickness and density of the oxide film, including formation of crystals, with more cohesion and bonding to the body metal than a single heating.

It should be understood that the specific procedure herein described merely represents a typical embodiment of the process characteristic of this invention, and the various adaptations and additions may be made in this particular manipulation or apparatus used without departing from the invention.

What I claim is:

1. The method forming a uniform and consistent oxide coating on a tantalum electrode closely bonded to a tough and non-brittle body which consists in moving said electrode a number of times into and out of a gas flame to cause intermittent exposure of the electrode to the atmosphere when heated to a duli red heat, Whereby the desired oxide is formed without causing the tantalum to absorb nitrogen to a degree to become brittle.

2. The method of forming a uniform and consistent dense coating of a dark colored oxide on the surface of a tantalum electrode for electron discharge tubes, which consists in repeatedly heating in a gas flame a portion of a thin tantalum sheet to a dull red heat and briefly exposing such heated portion to the atmosphere after each heating until the visible color disappears, said cycle of heating and brief exposure to the atmosphere being repeated for the diflerent portions of said tantalum sheet until an oxide coating of the desired density has been formed.

3. The method of forming on the surface of a tantalum electrode a uniform and closely bonded coating of a dark colored oxide of tantalum which consists in heating the various portions of the tantalum electrode to a dull red heat in the hot part of a gas flame adjusted to be slightly oxidizing, exposing said heated portions of the tantalum electrode to the oxidizing action of the atmosphere until the temperature has dropped to a level corresponding with disappearance of visible color of temperature, and repeating such alternate heating of the portions of the tantalum surface and exposure to the atmosphere until an oxide coating of the desired density has been formed over the entire surface.

EARL K. SMITH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,926,407 Ruben Sept. 12, 1933 1,940,029 Stansbury Dec. 19, 1933 2,070,691 Spencer Feb. 16, 1937 

2. THE METHOD OF FORMING A UNIFORM AND CONSISTENT DENSE COATING OF A DARK COLORED OXIDE ON THE SURFACE OF A TANTALUM ELECTRODE FOR ELECTRON DISCHARGE TUBES, WHICH CONSISTS IN REPEATEDLY HEATING IN A GAS FLAME A PORTION OF A THIN TANTALUM SHEET TO A DULL RED HEAT AND BRIEFLY EXPOSING SUCH HEATED PORTION TO THE ATMOSPHERE AFTER EACH HEATING UNTIL THE VISIBLE COLOR DISAPPEARS, SAID CYCLE OF HEATING AND BRIEF EXPOSURE TO THE ATMOSPHERE BEING REPEATED FOR THE DIFFERENT PORTIONS OF SAID TANTALUM SHEET UNTIL AN OXIDE COATING OF THE DESIRED DENSITY HAS BEEN FORMED. 